We plan to investigate the structure and rearrangements of the genes coding for immunoglobulin (Ig) heavy (H) chains in a murine B cell lymphoma (I.29) which appears to contain cells in the process of switching from the expression of IgM to IgA (Tada and Hammerling, submitted; Sitia and Hammerling, in preparation). The tumor cells synthesize IgM and IgA of the identical idiotype. By immunofluorescence, approximately 50% of the cells express both H chains simultaneously, and the others stain for either H chain alone. Clones of lymphoma cells expressing only IgA and hybridomas expressing either IgM or IgA have been obtained from the I.29 lymphoma. (1) I propose to perform restriction enzyme analyses of DNA isolated from these cell lines to determine if the CH switch (from Mu to Alpha chain) is accompanied by a translocation of C(H) genes. (2) We will determine if the cell lines derived from the I.29 lymphoma contain rearrangements of Ig H chain genes on both chromosomes, i.e. at both the expressed and excluded alleles, as has been found in plasmacytoma cells. (3) I plan to isolate the joined H chain genes from a cloned DNA library prepared from these cell lines in order to determine the arrangement of the Mu and Alpha chain genes. If these studies indicate that translocation of C(H) genes has occurred, we will determine the precise site of recombination of the DNA during the switch. (4) Cells synthesizing secreted IgA and cells synthesizing membrane-bound IgA have been obtained from the I.29 lymphoma. We will determine whether the secreted and membrane-bound IgA are coded for by different sized RNAs and if so, attempt to determine the cause of the size difference, whether it is by differential RNA processing, or by transcription from different transcriptic units, or a combination of both.